This invention generally relates to electronic systems, and more particularly it relates to power efficient line drivers.
Analog front-ends for Digital Subscriber Line (DSL) and Local Area Network (LAN) increasingly require to operate at lower supply voltages while larger output line driver voltages are required for operation on longer loops as well as enabling multilevel modulation for higher bit rate transmissions. At the lower supply voltage levels power efficiency is even more critical and hence 6 dB signal loss in terminating/matching resistors is significant. One solution to raise the output signal level would be to use a higher turn ratio transformer. The problem with this apart from the cost is that the receiver input noise specification would need to be reduced by the same ratio. Prior art References [1] B. Nauta and M. Dijkstra, xe2x80x9cAnalog Video Line Driver with Adaptive Impedance Matchingxe2x80x9d ISSCC Digest of Technical Papers, pp. 318-319, February 1998 and [2] Rajeevan Mahadevan and David Johns, xe2x80x9cA Differential 160 MHz Self-Terminating Adaptive CMOS Line Driverxe2x80x9d ISSCC Digest of Technical Papers, pp. 436-437, February 2000 discuss single-ended and differential methods of synthesizing the terminating impedances respectively. FIG. 1 shows the simplified prior art circuit in Reference [1]. Amplifier OP1 in conjunction with transistors M1 and M2, and resistor R2 provide an output impedance of R2/(1+N) due to the feedback. Amplifier OP3 forces input TXin and output TXout to be equal and hence sets the driver gain to one. Amplifiers OP1xe2x80x2 and OP3xe2x80x2, transistors M1xe2x80x2 and M2xe2x80x2, and resistor R2xe2x80x2 perform the same for the negative input. This ensures that the impedance matches that of the output line. Amplifier OP2 stops DC loss in the outputs, as the output common mode is forced to the same voltage. The circuit can be used as an impedance synthesizer by disabling the loop which forces TXin=TXout. This allows gain to be achieved in the driver stage.
The major draw backs of this method is the limited output swing, the extra trimming needed for the correct synthesized impedance, and the extra low frequency tuning loop needed to avoid DC loss in the termination.
Generally, and in one form of the invention, a line driver circuit includes: a differential circuit having first and second input nodes, first and second output nodes, and a supply voltage node; a transformer having a first coil coupled between the first and second output nodes; and a center tap of the first coil coupled to the supply voltage node.